U.S. President Donald Trump and German Chancellor Angela Merkel will meet ahead of the Group of 20 (G20) summit in Hamburg after having an "extensive" discussion on climate and trade issues by phone on July 3, their offices have said.

Merkel's spokesman Steffen Seibert said the meeting likely would be on July 6, the day before the summit, which Merkel is hosting.

Trump and Merkel had an "extensive discussion" on "climate issues, the Women's Entrepreneurship Financing Initiative, and trade, including global steel overcapacity," the White House said.

"The president said that he looks forward to helping Chancellor Merkel make the summit a success," it said.

Merkel predicted "a whole series of thorny issues" at the summit. The United States has departed from the group's previous consensus on climate change and trade since Trump took office.

"We know the positions of the U.S. government, and I do not expect them to disappear on a two-day trip to Hamburg," she said.

She warned against high expectations, noting that the summit's final communique had to be "approved unanimously."

She said that while there would be common ground on combating terrorism financing, "in other areas there are many differences."